1. Field of the Invention
The invention relates to a digital transmission system comprising a transmitter section which includes a coding arrangement and a receiver section which includes a decoding arrangement, at least a code translation matrix being provided between the input and the output of the coding arrangement and at least a code translation matrix being provided between the input and the output of the decoding arrangement and the transmission system comprising code conversion supervisory means.
2. Description of the Related Art
When digital signals, such as, for example, pulse code modulation signals are transmitted over large distances it is customary to use such a form of encoding in the transmitter section and such a form of decoding in the receiver section of the system that a transmission code is obtained which is adapted to the medium. This medium may, for example, consist of symmetrical, coaxial cables or optical fibres or the ether. One of the objects is usually the suppression of the d.c. component, so that alternating current couplings can be used in the system and it is possible to supply the regenerators with direct current from the system via the transmission cable. A further object is often the increase of the pulse density in the medium relative to the original binary code, or ensuring a minimum pulse density, as the case may be, so that the extraction of a clock signal, necessary for regenerative circuits, is possible.
Therefore, use is made in an encoding arrangement of what are commonly denoted as code translation Tables, for example the Tables described in Philips Telecommunication Review, Vol. 34, No. 2, June 1976, pages 72-86. Present at the input of the encoding arrangement is a series-parallel converter which divides the bit stream applied to its input into consecutive blocks having a fixed number of bits n, which consequently can occur in 2.sup.n combinations. So the length of the table is 2.sup.n. Thereafter an n-bit block is converted with the aid of the code translation matrix into a new block having m bits according to a predetermined instruction. At the output of the encoding arrangement the m-bit blocks are reconverted by means of a parallel-series converter into a bit stream which is conveyed to the receiving section of the digital transmission system via a medium (cable, ether). In the receiver section of the system an opposite operation is performed with the aid of the decoding arrangement on the bit stream applied thereto. Examples of an encoding arrangement and a decoding arrangement are described in, for example, Proceedings 17th International Scientific Congress on Electronics, Rome Mar. 16-18, 1970, pages 275-283.
In order to monitor a correct operation of the code conversion procedure, additional characters might be added to the binary data stream in time-division multiplex (T.D.M.) prior to encoding in the transmitter section of the transmission system. These additional characters are then removed in the receiver section after decoding from the binary data stream in time-division demultiplex (T.D.D.) and can be checked there for the rules added thereto by T.D.M. This method is often in T.D.M. line equipment having a fixed frame construction as a facility.
The above-mentioned method has the disadvantage that because of the additional TDM/TDD the signaling rate should be increased somewhat and furthermore the equipment required therefor would considerably increase the overall equipment and consequently the price and the dissipation of the system. It furthermore significantly reduces the reliability in operation.